High-frequency protective circuits



May 25, 1954 P. L. SPENCER HIGH-FREQUENCY PR OTECTIVE CIRCUITS FiledNov. 18, 1950 wdwh Patented May 25, 1954 UNITED STATES PATENT OFFICEHIGH-FREQUENCY PROTECTIVE CIRCUITS .Percy L. Spencer, West Newton,Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., acorporation of Delaware Application November 18, 1950, Serial No.196,434

8 Claims. 1

This invention relates to electrical circuits, and more particularly toa system for automatic pro tection of a microwave oscillator producingcontinuous waves.

In electron tubes used to produce microwave energy such as magnetrons,for greatest elliciency the device should work into a load of op timumvalue. When this is the case, the voltage standing wave ratio in thetransmission line connecting the tube and the load will be found to beunity. When this is not the case, the ratio will be at some highervalue. Thus the voltage standing wave ratio may be taken as anindication of match or mismatch of the load and generator. As thestanding wave is created by the presence of reflected energy'in thetransmission line, a device, such as a directive coupler adapted totransmit energy in one direction only and terminated in a detector, canbe used to detect the presence of standing waves in the transmissionline.

The effect of reflected energy on the magnetron is to bombard thecathode with electrons and heat it. If this condition is not correctedprompt- 1y, serious damage may be done to the tube. Further increase intemperature of the cathode may be prevented if the filament voltage ispromptly reduced.

It is the purpose of the present invention to provide means for sensingan increase in the refiected power and in response reduce the filamentpower. This is done by inserting a directive coupling in thetransmission line and deriving from it a control voltage that in turncontrols the current through a saturating winding on the filamenttransformer supplying filament power to the magnetron.

Such a control system automatically maintains the cathode of the tube atthe optimum temperature in spite of considerable variation in loadconditions. Thus the cathodes and other structures of the tube are notexposed to excessive heat for any appreciable length of time, thusprolonging the life of these relatively expensive tubes. Other automaticcontrol devices for micro-wave tubes, such as that disclosed in PatentNo. 2,498,719, depend upon critical spacing of one or more probes withinthe transmission line from generator to load. The present device isindependent of the phase of the standing wave and so does not have to bepositioned at any particular point in the transmission line.

These features make the device particularly useful in industrialapplication of microwave energy where the energy is used to heat varioussubstances presenting various loads to the generator. It is also usefulin unattended microwave communication links, such as television relayswhere the load conditions may change suddenly in the absence of askilled operator.

Other and further advantages of this invention will be apparent as thedescription thereof progresses, reference being had to the accompanyingdrawing, wherein the single figure is a diagrammatic representation of aprotective system according to this invention.

Now referring to the drawing, the numeral I0 refers generally to themagnetron generating microwave energy that is picked up by the an.-tenna probe I I and introduced into a wave guide I2. At a convenientdistance a loop I3, of the type described by H. C. Early in theNovember, 1946, Proceedings of the Institute of Radio Engineers at page883 and shown schematically in Fig. 1 of that article, and in greaterdetail in Figs. 6 and '7, is inserted into the wave guide I2 through aslot I4. One side of the loop may be connected through a coaxial cableI5 to an incident power indicator, not shown as it is no part of thepresent invention. The far side of the loop is connected through alength of coaxial cable I!) to a crystal rectifier [6 that is connectedto ground through a resistor I1. The crystal I6 is connected in suchpolarity that, as the reflected power picked up by the loop I3 becomesgreater, the junction point I8 becomes increasingly negative. The pointI8 is connected to an amplifier 20 which may be of any conventionaldesign adapted to amplify a slowly varying D. C. potential. The inputshould be of high impedance and the output of low impedance. This outputis connected in series to a saturating winding 2| on the core 22 of afilament transformer 23. The primary 24 of this transformer is connectedto a source of A. C. power 25. The secondary 26 is connected to thefilament 2'! of the magnetron I0.

When the magnetron I0 is operating normally into a matched. load, thevoltage standing wave ratio will be unity and there will be no reflectedpower to be picked up by the loop I3 and detected by crystal IS. Theamplifier 20 passes a normal amount of current through the winding 2|leaving the core 22 unsaturated and permitted the magnetron filament 21to receive its normal amount of energy.

When the load changes to create a mismatch and cause reflected energy tobe picked up by the loop I3 and detected by the crystal I6, the flow ofcurrent through the saturating winding 2| increases to saturate the core22 of the filament transformer 23. This reduces the amount of powertransferred to the secondary 26 and delivered to the filament 21 of themagnetron, thus reducing the temperature of its cathode.

When the load conditions return to normal, the standing wave ratio in thtransmission line i2 returns to unity and there is no reflected energyto be detected and no signal at the input of the amplifier 20. Thisreduces the flow of current through the saturating winding 2| on thecore 22 of the transformer 23 permitting full heater energy to pass tothe filament 21 of the magnetron in increasing its temperature, thusmaintaining the cathode of the magnetron H3 at a constant temperatureunder a wide range of load conditions.

Any type of directional coupler could be used in place of the loop l3shown. Also any con venient type of rectifier could be used in place ofthe crystal type I6 shown.

This invention is not limited to the particular details of constructionand materials described, as many equivalents will suggest themselves tothose skilled in the art. It is accordingly desired that the appendedclaims be given a broad interpretation commensurate with the scope ofthe invention within the art.

What is claimed is:

l. A protective system comprising a source of radio frequency energy, aheater for said source, means for delivering energy to said heater, atransmission line for coupling said source to a load, means fordetecting reflected energy in said transmission line, and means forvarying the energy delivered to said heater in response to the output ofthe detecting means.

2. A protective system comprising a source of radio frequency energy, aheater for said source, means for delivering energy to said heater,comprising a transformer with a core and a saturating winding on saidcore, a transmission line for coupling said source to a load, means fordetecting reflected energy in said transmission line, and means forvarying the energy delivered to said heater in response to the output ofthe detecting means, comprising a saturable core reactance in said meansfor delivering energy to said heater.

3. A protective system comprising a source of radio frequency energy, aheater for said source,

means for delivering energy to. said heater, comprising a transformerwith a core and a saturating winding on said core, a transmission linefor coupling said source to a load, means for detecting reflected energyin said transmission line,

and means for varying the energy delivered to a said heater in responseto the output of the detecting means, comprising means for coupling theoutput of said detector to the said saturating Winding.

4. A protective system comprising a source of radio frequency energy, aheater for said source, means for delivering energy to said heater, atransmission line for coupling said source to a load, means fordetecting reflected energy in said transmission line, said meanscomprising a rectifier coupled to said transmission line through adirectional coupling means, and means for varying the energy deliveredto said heater in response to the output of the detecting means.

5. A protective system comprising a source of radio frequency energy, aheater for said source, means for delivering energy to said heater,comprising a transformer with a core and a saturating winding on saidcore, a transmission line for coupling said source to a load, means fordetecting reflected energy in said transmission line, said meanscomprising a rectifier coupled to said transmission line through adirectional coupling means, and means for varying the energy deliveredto said heater in response to the output of the detecting means,comprising a saturable core reactance in said means for deliveringenergy to heater.

6. A protective system comprising a source of radio frequency energy, aheater for said source, means for delivering energy to said heater,comprising a transformer with a core and a saturating winding on saidcore, a transmission line for coupling said source to a load, means fordetect- 25 ing reflected energy in said transmission line, said meanscomprising a rectifier coupled to said transmission line through adirectional coupling means, and means for varying the energy deliveredto said heater in response to the output of .0 the detecting means,comprising means for coupling the output of said detector to the saidsaturating Winding.

'7. A protective system comprising a source of radio frequency energy,an energy control for 35 said source, means for delivering energy tosaid energy control, a transmission line coupling said source to a load,an alternating current directional coupler connected to saidtransmission line for detecting substantially only reflected energy 4.0in said transmission line, and means for varying said energy control inresponse to the output of said detecting means.

8. A protective system comprising a source of radio frequency energy, anenergy control for said source comprising a saturable core reactance,means for delivering energy to said energy control, a transmission linecoupling said source to a load, an alternating current directionalcoupler connected to said transmissionline for detecting substantiallyonly reflected energy in said transmission line, and means for varyingsaid energy control in response to the output of said detecting means.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,930,545 Wensley Oct. 17, 1933 2,149,080 Wolff Feb. 28, 19392,175,694 Jones Oct. 10, 1939 2,236,195 McKesson Mar. 25, 1941 2,262,044Philpott Nov. 11, 1941 2,386,040 Edwards Oct. 2, 1945 2,408,091 OlesenSept. 24, 1946

